Pressure sensor with integrated structure

ABSTRACT

Pressure sensor with an integrated structure comprising a support ( 33, 133, 233 ) and a silicon die ( 11 ), which lies substantially on the same plane as the upper surface ( 37, 137, 237 ) of the support and is integrated in a seat ( 34, 134, 234 ) made in the thickness of the support. On the inside face ( 23 ) of the die ( 11 ), in contact with the fluid the pressure of which has to be measured, is a protective layer.

TECHNICAL FIELD

This invention relates to a pressure sensor with an integratedstructure.

More, specifically, the invention relates to an integrated pressuresensor, suitable for use in hostile environments, comprising a silicontransducer assembled together with a support, as described in the mainclaim.

BACKGROUND ART

Of the many known types of pressure sensors, silicon sensors have hadgreat market success, thanks especially to their reliability, low costand compact dimensions.

The methods of production and operation of silicon sensors are known,and for this reason will not be described in detail herein, while onlysome characteristics important to the understanding of the inventionwill be described in greater detail.

Depicted in FIG. 1 is a silicon pressure sensor (10), constructedaccording to the known art: a silicon transducer or die (11) comprises athin and flexible membrane (14), made by removing part of the silicon,for instance by chemical etching or by mechanical detachment; made usingknown techniques on an outer face (24) of the die, are piezoresistors(21) that are connected together so as to form a Wheatstone bridge.

When pressure is applied to the transducer (11), this causes themembrane (14) to flex, as a result of which the resistance of thepiezoresistor (21) changes; in this way, when a voltage is applied tothe circuit, a change in the current may be read when pressure isapplied to the sensor.

The silicon transducer or die (11) is then glued, using for instance asilicon- based adhesive, to a plate (12), made of a material, pyrex,silicon or glass for example, that has a thermal expansion coefficientclose to that of the transducer (11). When the sensor (10) is subjectedto a change in temperature, the plate (12) undergoes an expansionsimilar to that of the die (11), thus avoiding stresses being broughtabout on the transducer (11) by a difference in expansion between thetransducer (11) and the plate (12), which would cause read errors.

In turn the plate (12) is joined, using known techniques, to a support(13), which may be, for instance, of ceramic material or consist of aprinted circuit.

Built on the support (13) is an electronic circuit, which is connectedby way of bonding wires (22) to the piezoresistors (21) and whichcomprises components for amplifying and correcting the signal output bythe sensor (10); the signal is in turn sent for reading to a boardexternal to the sensor (10), not shown in any of the figures.

In many of the applications of pressure sensors, they have to be used ina hostile environment, which may for example consist of a fluidcontaining corrosive substances, or which is at high pressure ortemperature.

In order to protect these sensors a resin shell is widely used (25),made using techniques known in the assembly of electronic components.

To allow the fluid to come into contact with the transducer (11),apertures (15) and (16) are made in the plate (12) and on the support(13), whilst an aperture (27) is made in the shell (25) that puts thesensor into communication with the outside, thus permitting thedifference in pressure with the environment to be measured.

The piezoresistors (21) and the bonding wires (22) that are on the outerface (24) of the transducer (11) could be damaged by the action of anaggressive fluid; on the contrary, on an inner face (23) of thetransducer (11) there are no delicate components and therefore there areno particular problems if the inner face (23) is in contact with thefluid.

For this reason, sensors have been produced for hostile environments,that are operated in such a way that the transducer (11) has its innerface (23) in contact with the aggressive fluid. In addition, the surfaceof the inner face (23) may be covered by a protective layer, made forinstance of a layer made of alloys of chromium, tantalum, siliconcarbide or others.

In these sensors, the outer face (24) of the die (11), which comprisesdelicate components such as the piezoresistors (21) and the bondingwires (22), is not in contact with the aggressive fluid.

In addition the transducer (11), the piezoresistors (21), the bondingwires (22), and the support (13), upon which the tracks of theelectronic circuit are made, may be protected even further by coveringthem with a gel or a protective resin (26).

Should the fluid the pressure of which has to be measured, act on theopposite face of the sensor, i.e. the outer face (24), therefore passingthrough the aperture (27) of the shell (25), the gel (26) performs thefunction of protecting the outer face (24) of the transducer (11), withthe piezoresistors (21) and the bonding wires (22), from the action ofthe same fluid.

In the latter case however, the gel (26) does not guarantee the surfaceof the outer face (24) of the die (11) a sure protection, as it may bedamaged by the action of the aggressive elements present in the fluid.

The silicon sensors for hostile environments made according to the knownart have a number of drawbacks: the protections needed for applicationsin hostile environments complicate construction of the sensor, requiringassembly of a large number of parts and sometimes the production ofcostly fixtures.

Another problem, where there are high pressures and an aggressiveenvironment, may be caused by the join between the transducer (11) andthe plate (12), which may not be resistant enough to support highstresses.

Moreover the application in hostile environments may give rise toapplied tensions on the transducer (11), caused by sudden jumps intemperature or by transitory extra-tensions induced on the support (13)by the fluid itself, with resultant read errors. In the sensor (10) madeaccording to the known art, the plate (12) and the transducer (11) areglued above the support (13), and are therefore protruding with respectto the surface of the support

This assembly leaves the transducer (11) more exposed to the action ofexternal transitory forces, caused by the fluid itself, which can causemomentary incorrect pressure value readings or even damage thetransducer.

SUMMARY OF THE INVENTION

The object of this invention is to produce a pressure sensor, forapplication in hostile environments, comprising a small number of easyto produce parts, which is easy to assemble, and therefore low cost.

A second object is to produce a sensor, comprising a transducer that isnot subject to stress caused by deformations induced by external forces.

A third object is to produce a sensor, comprising a transducer that isprotected if used in hostile environments.

A fourth object is to produce a sensor that is resistant if used formeasuring high pressures.

These objects are achieved by the pressure sensor with integratedstructure of the invention in accordance with the characteristic partsof the main claims.

These and other characteristics of the present invention will becomeapparent in the course of the following description, provided by way ofnon-restrictive example, with the aid of the accompanying drawings.

LIST OF FIGURES

FIG. 1 is a section view of a silicon pressure sensor according to theknown art;

FIG. 2 is a section view of a pressure sensor with integrated structureaccording to this invention.

FIG. 3 is a section view of a pressure sensor with integrated structureaccording to a second embodiment of the invention.

FIG. 4 is a section view of a pressure sensor with integrated structureaccording to a third embodiment of the invention.

DESCRIPTION

First embodiment

With reference to FIG. 2, a pressure sensor with integrated structure orsensor (30), according to the invention, consists of a transducer or die(11), having an upper edge (28), made of silicon according to the knownart, a support (33), having an upper surface (37), and a-container (35).

The support (33) is made, for instance, of ceramic, or alternatively mayalso be made of a known type of printed circuit, and is glued to thecontainer (35) by means of an adhesive (41), of the epoxy type forexample. Made on the inner surface of the container (35) is a step (43)permitting a better gluing of the support (33).

Inside the support (33) is a seat (34), obtained from a mould or by amachining operation, in which the die (11) is integrated. To be able tohouse the die (11), the seat (34) must have the same profile as the die(11), but dimensions greater than those of the die.

The seat (34) may for instance have a width of about 0.1÷0.2 μm greaterthan that of the die (11), so as to create a space in which to insert anadhesive (40) for gluing of the die (11) to the support (33). Theadhesive (40) may be silicon type, for instance, so that any tensionsinduced by the support (33) are not transmitted to the die (11).

The die (11) is then integrated in the seat (34), made in the thicknessof the support (33) and is assembled in such a way that the upper edge(28) of the die (11) is substantially on the same plane as the uppersurface (37) of the support (33).

The sensor (30) may for instance be screwed or joined by snap means, andis installed in such a way that the fluid the pressure of which has tobe measured enters through an inlet (45), located in the top part of thecontainer (35).

When the sensor (30) works in hostile environments, for example incontact with a fluid that may contain aggressive substances or be athigh pressures or temperatures, the die (11) and the support (33) needto be protected.

For this reason the inner face (23) of the die, in contact with thefluid to be measured, is protected by means of known techniques, forinstance by placing a layer made of chromium, tantalum, silicon carbideor other alloys on top.

Moreover, to protect the support (33) from attack from the fluid, itsupper surface (37) may be, for instance, vitrified so as to make itimpermeable to the action of the fluid.

In the sensor (30) made according to the invention, the bonding wires(22) and the piezoresistors, which are more fragile and could be damagedif coming into direct contact with the fluid, are on an outer face (24),opposite to the fluid to be measured, which is in contact with theexternal environment.

The bottom surface (36) of the support (33) also faces the outside, andon it a thick film circuit, not depicted in the figures, is produced andis connected to integrated or programmable electronic components (46)(IC and ASIC), used for amplification and adjustment of the sensor readsignals.

For better protection of the thick film circuit, the electroniccomponents (46), the bonding wires (22) and the piezoresistors of thedie (11), they may be coated with a layer of silicon resin (42) which,being elastic, is capable of following the deformations of the membrane(14), and does not therefore influence the pressure measurement effectedby the die (11).

Between the support (33) and the container (35), in the area filled bythe adhesive (41), small feet (44) are inserted for electric connectionof the sensor with an external circuit.

With the sensor produced according to the invention, a device can bemanufactured using a small number of parts, and that is easier tomanufacture than those of the known art.

In fact, with respect to the known art, the die (11) is no longer gluedon a plate (12) (FIG. 1) but directly on the support (33), integrated inits thickness and less operations, less parts are required.

In addition, as the upper edge (28) of the die (11) is substantially onthe same plane as the upper surface (37) of the support (33), the die(11) remains protected from stresses from the outside that could causeextra stress, and possibly read errors or even the die (11) becomingdetached from the support (33).

Second embodiment

Described in FIG. 3 is a second embodiment of the sensor (30″) accordingto the invention.

In the vicinity of the upper surface (137) of the support (133), in theseat (134), a step (138) is made that runs along the edge of the seat(134) itself.

The seat (134) has the same profile as the die (11), but its dimensionsare greater than those of the die, in such a way as to leave a spacefree between the support (133) and the die (11) in which to insert theadhesive (40) for gluing.

The seat (134) may for instance have a width of about 0.1÷0.2 μm greaterthan that of the die (11).

The step (138) is made local to the upper surface (137) of the support(133), which is the part of the support that comes into contact with thefluid, the pressure of which has to be measured.

The die (11) is thus integrated in the seat (134), made in the thicknessof the support (133) and is assembled in such a way that the upper edge(28) of the die (11) is in abutment with the step (138).

The surface of the part of the step (138) in contact with the upper edge(28) of the die (11), is lesser than the, surface of the upper edge (28)itself; in this way, a part of the surface of the upper edge (28)remains available to be coated by the adhesive (40), and the risk of theadhesive (40) spreading on to the membrane (14) during gluing, causingread errors, is avoided.

The sensor (30″) made according to the second embodiment of theinvention has the advantage that it permits more precise positioning ofthe die (U) on the support (133).

In addition, the sensor (30′) improves the gluing between die (11) andsupport (134), since the surface of the die (11) concerned by theadhesive is of greater extent than that of the sensor of the firstembodiment of the invention: the die (11) is glued, as well as on thelateral surface, also along a part of the upper edge (28).

In addition, the step (138) protects the transducer (11) from stressesthat could come from the outside and cause extra stress, with possibleread errors or even the die (11) becoming detached from the support(133).

Third embodiment

In FIG. 4 a third embodiment of the sensor (30″) according to theinvention is described.

The sensor (30″) described has a step, which runs along the outerprofile of the seat (234), and is made between the lower edge of theseat (234) and the lower surface (236) of the support (233).

The seat (234) is made on the inside of the support (233) and has, inthe vicinity of the lower surface (236) of the support, a step (238)that runs along the edge of the seat (234).

The seat (234) has the same profile as the die (11), but its dimensionsare greater than those of the die, in such a way as to leave a freespace between the support (233) and the die (11) in which to insert theadhesive (40) for the gluing.

The seat (234) may for instance have a width of about 0.1÷0.2 μm greaterthan that of the die (11).

The step (238) is made local to the lower surface (236) of the support(233), which is the part of the support opposite that in contact withthe fluid of which the pressure has to be measured.

The die (11) is therefore integrated in the seat (234), made in thethickness of the support (233), and is assembled in such a way that itsouter face (24) is in abutment with the step (238).

The surface of the outer face (24) of the die (11) in the sensor (30″)made according to the third embodiment of the invention, is greater thanthat of the die used in the sensors (30 and 30′), made according to thetwo embodiments described earlier. In this way, a part of the surface ofthe upper edge (28) is available to be coated by the adhesive (40), andthe risk of the adhesive (40) spreading on to the membrane (14) duringgluing, causing read errors, is avoided.

Part of the area of the outer face (24) is in contact with the step(238). In order to have the same free surface as in the sensorsdescribed in the first two embodiments (30 and 30′), the area of theouter face (24) of the die must therefore be greater than in the firsttwo sensors (30 and 30′).

For membranes of like dimensions, the die (11) must be slightly broader(+0.2 μm per side), so that the surface available for the pads and theresistors is the same as in the sensors (30′ and 30″), made produced tothe first two embodiments of the invention.

In addition to the advantages that are given by the first twoembodiments, presence of the step (238) below the die (11) means that asensor is obtained that can resist the stresses that are generally foundin the presence of high pressures.

1. Pressure sensor with an integrated structure comprising: a silicondie, having an upper edged, an inner face, and an outer face on whichpiezoresistors are mounted, a support having an upper surface, and aseat suitable for containing the die, a container suitable forimplementation in such a way as to put the die in contact with the fluidthe pressure of which has to be measured, wherein said die is integratedin said seat, made in the thickness of the support.
 2. Pressure sensorwith integrated structure according to claim 1 wherein the seat passesthrough the support
 3. Pressure sensor with integrated structureaccording to claim 1 wherein a step is made on the inner surface of thecontainer, enabling improved gluing of the support.
 4. Pressure sensorwith integrated structure according to claim 1: wherein the upper edgeof the die is substantially on the same plane as the upper surface ofthe support.
 5. Pressure sensor with integrated structure according toclaim 1: wherein the inner face of the die in contact with the fluid thepressure of which has to be measured.
 6. Pressure sensor with integratedstructure according to claim 1: wherein on the inner face of the die, incontact with the fluid to be measured, is a layer of protection madefrom chromium, tantalum, silicon carbide alloys.
 7. Pressure sensor withintegrated structure according to claim 1: wherein in the vicinity ofthe upper surface of the support, on the inside of the seat, a step ismade that runs along the edge of the seat itself.
 8. Pressure sensorwith integrated structure according to claim 7 wherein the die isassembled in such a way that the upper edge of the die is in abutmentwith the step.
 9. Pressure sensor with integrated structure according toclaim 7: wherein the surface of the part of the step, that is in contactwith the upper edge of the die, is less than the surface of the upperedge itself.
 10. Pressure sensor with integrated structure according toclaim 1: wherein in the vicinity of the lower surface of the support, onthe inside of the seat, a step is made that runs along the edge of theseat itself.
 11. Pressure sensor with integrated structure according toclaim 10: wherein the upper edge of the die is substantially on the sameplane as the upper surface of the support.
 12. Pressure sensor withintegrated structure according to claim 10: wherein the die is assembledin such a way that its outer face is in abutment with the step.